Clamp for metal stretching presses



Feb. 22, 1944 I o. M. SUMMERS 2,342,437

CLAMP FQR METAL STRETCHING PRESSES Filed Nov. 13, 1940 2 Sheets-Sheet 1 QR v NVENTOR Ono Muakav ans Patented Feb. 22, 1944 UNITED STATES PATENT OFFICE CLAM? FOR METAL STRETCHING PRESSES Otto M. Summers, Mount Gilead, Ohio, assigncr to The Hydraulic Development Corp. Inc., Wilmington, Del., a corporation of Delaware Application November 13, 1940, Serial No. 365,536

Claims.

This invention relates to presses of the type that employ mechanism for clamping the workpiece to the press.

Certain kinds of sheet metal articles are economic'ally fabricated by means of a stretching press in which a plain sheet of metal is bent at the proper places and stretched at other places until the desired configuration of the article is obtained. It is necessary to clamp the metal sheet to the press about the periphery of the sheet, and in the case of complicated forms of workpiece it may be necessary to permit slippage of the metal to take place in order to provide additional metal at the stretching position.

It has been customary heretofore, in order to provide these combined clamping and slippage effects, to employ clamping cylinders about the main ram and to apply individually to these cylinders pressure fluid, preferably from thesame source as supplies the main cylinder. In order to effect the necessary slippage at one point without affecting a strict clamping effect at another point, the pressureofthe fluid admitted to the several cylinders Was varied by separate control valves. However, it has been found that this method of obtaining a clamping effect at one clamping cylinder and a slippage eifect at another clamping cylinder was not altogether satisfactory, because when slippage was permitted by reducing the clamping effect at its individu'al cylinder the lightening of the clamping pressure at this point tended also to reduce the clamping efiect at the instant cylinder so that the-clamping and slippage effects were not'individually under rigid control. Moreover, the

clamping rams of the prior art presse were sometimes provided with a flat face where the ram contacts the workpiece, and in order to eliminate even the slightest amount of slippage the pressure applied to the clamping mm was exceedingly high.

The primary object of the present invention is to provide an improved metal stretching press,

and more particularly to improve the character of the clamping jaws with a view to obtaining more rigid control over the clamping and slippage eiiects of the metal, whereby metal articles may be formed on the press exactlyto desired shape and size.

Another object is to provide a clamping mechanism for stretching presses, and in which the uneven clamping force necessary to bring about one hundred per cent. clamping at one point and a predetermined amount of slippage at another point is transmitted .to theworkple ce with a smoothly graduated intensity, thus pre- Venting twisting or wrinkling of the metal at positions between the clamping and slipping jaws.

Still another object of the invention is to provide an improved clamping jaw in which positively not the slightest amount of slippage takes place, unless otherwise desired, and utilizing only moderate clamping pressures which are readily available at the press.

An additional object is to provide an improved combined clamping and slippage jaw in which the transition from one hundred per cent. clamping elfect to one hundred per cent. slippage effect takes place gradually and in a predetermined manner, so that any degreeof slippage between these limits can be readily and accurately obtained.

The above objects are obtained, in brief, by providing individually operated pairs of clamping jaws, these jaws having smooth corrugated clamping surfaces which form similar corrugation in the edges of the workpiece so that the latter is held between the jaws throughout a serpentine path, in which any desired amount of friction may be obtained, by simply changing the pressure applied to the corrugated jaws.

An added feature consists of providing oneof the corrugated jaws with a projection which bites only slightly into the workpiece or metal blank regardless of the pressure that might be applied to the clamping jaws, and the purpose of which is to exert a small but very effective gripping action on the workpiece before it has passed through the serpentine or corrugated path.

From still another aspect, my invention contemplates the use of spring or other flexible members joining the clamping jaws together in such a way that when one jaw is regulated to provide a slippage effect, and the pressure on the adjacent jaw is such as to provide one hundred per cent. clamping effect, the difference in clamping pressure as applied to the workpiece is graduated and no abrupt change in the clamp ing pressure at any predetermined point along the workpiece is permitted.

Other objects and features will be apparent as the specification is perused in connection with the accompanying drawings.

In the drawings:

Figure 1 shows, in diagrammatic form, a stretching press provided with clamping mechanism which has been improved in accordance with the present invention.

Figure 2 .is an enlarged fragmentary view :of

I which a uniform gradation of clamping pressure is obtained between adjacent clamps set respectively for the two extreme clamping conditions.

Figure 4 is an enlarged perspective view of the flexible strips of metal employed to provide the graduated clamping effects. The ends of these strips are shown in section.

Figures 5 and 6 are diagrams to show by comparison the advantages obtained by flexibly connecting the clamping rams together in order to produce a smooth gradation of the clamping pressure between adjacent clamps.

Figure 7 is a sectional view taken through a pair of improved clamping jaws, the latter being moved slightly apart to permit a limited and predetermined amount of slippage.

Figure 8 is a view similar to Figure '7, but with the clamping jaws brought together to the extent that no slippage whatsoever is permitted.

Referring more particularly to Figure 1, the numeral I designates the main cylinder of a hydraulic press, the cylinder being closed at the bottom by the metal of the cylinder. The upper end of the cylinder is spanned by a closure plate 2, which carries a packing gland 3. The cylinder interior receives a reciprocating piston 4 forming part of a plunger 5, and on which rests a ram or platen 6 having a rounded surface at its upper end to accommodate the workpiece 1 being stretched. The cylinder 1 is secured in any suitable manner to a base plate 8 carried on'the machine frame 9.

The metal sheet or workpiece l is secured at its outer edges around the periphery by means of the improved clamps, which are generally designated l0 and which will be described presently. These clamping devices slidably rest on a pair of oppositely disposed plates H which are carried on the base plate 8, the clamps being secured to the base plate by the screw members l2 which pass through the base plate 8 and the top plates H. The screw members I2 are contained in slots which permit these members to be moved transversely of the base plate, and to carry with them the clamping mechanism It in order to accommodate different widths or lengths of workpiece. This transverse adjustment of the clamping devices may be effected in any suitable manner but as illustrated there is employed a diagonally positioned screw l3 with a hand wheel M at one end, the opposite end of the screw being contained in a swivel anchor 15. The screw 13 passes through a lug on the clamping member I0, the arrangement being such that as the hand wheel I4 is rotated, and assuming that the upright screw members l2 are loosened, the clamping devices are caused to slide along the upper surface of the plate I l to any desired position.

The improved clamping jaws are shown more clearly in Figures 7 and 8, and it will be noted that they are constituted of two mating blocks I6, I! having relatively large rounded clamping teeth as corrugations, and in which the recesses of one corrugated portion are directly opposite of the projections of the opposite corrugated portion. The surfaces of the corrugated parts are preferably smoothly graduated except that one of the mating blocks, the element ll as illustrated, is provided with a relatively sharp but smooth biting surface, indicated at 8, and located to the rear of the rounded teeth. This biting portion is preferably of rounded contour, but the radius of the curvature is considerably less than the curvature of the rounded teeth so as to effect a biting or pinching action without cutting the metal of the workpiece.

The block l6 preferably has a rectangular con figuration, with its lower corners chamfered, and is securely held in a snug fit within a slot indicated at [9 within a heavy jaw-shaped member 20, which is provided with an upstanding boss 2| through which the threaded screw member l3 projects. It has been pointed out that by turning the hand-wheel [4 the jaw-shaped member can be slid along the upper surface of the plate II. The jaw member 20 is provided with a transversely extending slit 22 in order to receive the edge of the workpiece 1. The corrugated block I! which mates with the corrugations on the block I6 is slidably secured to the jaw member 20 on the opposite side of the slot 22 from its mating block. The block I! is carried at opposite sides between a pair of guide plates 23 which are secured, as by screws 24, to the jaw member. The latter is provided with a portion 25 which extends to the right, as seen in Figure 3, this portion being hollow and constituting a cylinder for receiving an hydraulically operated plunger 26. -The latter is provided with piston rings 21 and terminates in a relatively wide cylindrical portion 28, to which is secured a platen 29 having a configuration similar to that of the block l1. While the platen 29 could be secured directly to the block I1, I prefer to interpose between these members a flexible and yieldable connecting piece 30, the details of which are shown in Figure 4. The purpose of this yieldable member 30 will be explained in connection with Figures 5 and 6.

Referring now to Figures 2-and 4, assume that there areas many jaw members 20 distributed around the workpiece as are necessary to hold the latter against slippage during the stretching operation. Five such units are diagrammatically illustrated in Figure 2. The connecting piece 30 spans all of the blocks I1, and is constituted of a number (five as illustrated) of U-shaped metal retainers 3! of thin springy material which hold in place a plurality of heavy rounded edge spring bars 32. The rounded edges are indicated at 33. Two such bars 32 are provided for each U-shaped flexible member 3|, these bars being separated from one another at their inner surfaces by a number of capscrews 34, equi-distantly spaced throughout the length of the bars 32 and threaded into the block l1. One end of each rod 34 is threaded into an opening provided in the block I1, and the opposite end of each rod carries a head 35 provided with a hexagonal countersink 36 for tightening the threaded end of the rod into the block, as can be clearly seen in Figure 3. It will be noted that the elements 3|, 32 are secured together solely by the clamping efiects of the capscrews 34, and due to the yieldable character of the members 3| the connecting piece as a whole constitutes a composite spring-like strip which yieldably holds all of the blocks ll of each jaw member 29 in alignment. The length of each bar 32 is approximately the length of each block l'l, so that if the latter are spaced a predetermined distance apart, the blocks 11 are held together only by the spring-like strips l3 and over the distance between each adjacent pair of blocks.

As seen in Figure 2, there is provided a triangularly shaped plate member 31, screwed at each corner to the adjacent members H, the lower apex of the trangular member being apertured to receive a tension spring 38 which is secured to any convenient part of the press, as indicated at 39. The purpose of the spring 38 is to retract the movable block I! when clamping pressure is relieved from behind the plunger 26. As shown in Figure 3, a pressure fluid port 40 is provided to the right of the plunger and a drainage port 4| is provided to the left of the plunger. Consequently, pressure fluid could be introduced at the port All, in a manner which will be described presently, causing the plunger 26 to move to the left and carrying with it the movable block or jaw ll.

Figure 1 shows a suitable and typical hydraulic system for operating the clamping jaws simultaneously with the operation of the main ram 5. In this figure, the reference character 42 designates the main variable delivery pump, of any suitable and standard type, including a flow-control member or shift ring (not shown). This shift ring is moved either onto or away from neutral by means of a flexible shaft 43 which is mechanically connected to a hand wheel 44 secured to a panel 45. Pressure fluid is contained in a reservoir, the latter communicating with the pump through a conduit 47. The discharge of the pump is taken through a conduit E8, into a four-way control and reversing valve 49, of any suitable and Well known construction. The valve stem 50 is secured at one end to a tension spring and at the other end terminates in a movable core 52 of an electromagnet 53. The latter is connected through the conductors 54 to a pushbutton switch 55, mounted on the panel &5. The reversing valve 49 is provided with a pair of conduits 56, 51, the former passing to the upper or retraction side of the main piston 4 and the latter being connected to the lower or forward side of the piston. Suitable conduits 58, 59 are taken from the valve 49 to the variable delivery pump 42.

The arrangement is such that if the hand wheel M is rotated to shift the flow-control member of the pump :12 away from neutral,

causing fluid to pass upwardly through the pipe 48, and assuming that the push-button 55 has been pressed to draw the valve stem 59 to the right, pressure fluid is caused to pass through the valve, through theconduit 51 to the lower or forward side of the piston 4. The latter is therefore forced upwardly, pushing the platen 6 against the workpiece l. The valve 49 causes the piston s to remain on stroke as long as the push button 55 is pressed, but when the latter is released the spring 5i reverses the valve 3-9 to permit pressure fluid to pass upwardly through the pipe 55, and to connect the conduit 5! with the exhaust port of the pump. Thus, by operating the button 55 the upward and downward movements of the piston A may be readily controlled, assuming that the hand wheel 44 has been rotated to place the pump 42 on stroke.

Pressure fluid for the clamping pistons 26 is obtained preferably from the pump 60, which is separate from the main pump 42 but draws its fluid from the same reservoir 46. The pump Gil is also of the variable delivery type, and the discharge line therefrom is indicated at 6|. It will be understood that there are as many pressure lines 5| 'as the number of clamping cylinders 25, extension conduits 62 being taken from the main line 6| to the left-hand group of cylinders. Conduits 63 communicate at one end with the pressure ports 40 (Figure 3) and at the other end are connected through a so-called constant pressure valve 64, the inlet side of which is connected to the pressure line Hi. The valve 54 is controlled preferably through a flexible shaft by the hand wheel 66 on the panel Q5.

The valve 6 3 may be of any suitable and well known type in which a drop in pressure is prcduced by rotating the shaft E5. The valve, as illustrated, requires at least a minimum flow of fluid therethrough in order to provide the necessary drop in pressure, and the amount of fluid continually passing through the valve is controlled by a hand-operated relief valve El which fits in to a pipe 63, the latter in turn being connected to a fluid exhaust pipe 65. It will be understood that there are as many pressure control valves 64 as there are clamping cylinders, and these valves can all be controlled either by a single hand wheel 66 or, as illustrated, they may be controlled in right-hand or left-hand groups in which the right-hand group is regulated at the hand wheel 66 and the left-hand group controlled by the hand wheel 19, through the flexible shaft ll.

It will be noted that there is no provision made for interchanging the connections at the pump 8%] between the pump lines SI and til, so that in the clamping cylinders pressure fluid is admitted only to the right-hand end of the piston 2t (Fi ure 3), and the springs 38 are relied upon to retract the clamping pistons when pressure fluid is Icy-passed around the clamping cylinders at the valve 5?. Instead of providing an individual valve fi l for each clamping cylinder, two or more of the latter may be arranged to receive pressure fluid through a single valve. Such an arrangement is shown in Figure 2, in which the first pair of cylinders obtains its pressure fluid through a single pressure pipe 72, which is provided with a single constant pressure valve seand a single relief valve 61. The same arrangement has been shown in connection with the next pair of cylinders, but the last cylinder of the group of five receives its pressure fluid through its own constant pressure valve 54 and employs its own relief valve 51. Thus, in the case of the first two pairs of cylinders, each cylinder of the pair is controlled in common with the other cylinder while the pressure fluid-in the last cylinder is controlled by an individual valve.

The reason for controlling the various cylin. ders either in multiple or individually by separate valves is that one or more of the clamping pistons may have a pressure applied thereto which permits slippage of the workpiece I, while the remaining clamping pistons might be pressed on the workpiece so hard as to prevent even the slightest amount of slippage. Thus the points at which one hundred per cent. clamping eifect and one hundred per cent. slippage effect over the surface of the workpiece can be accurately controlled, and any degree of clamping between these limits may also be obtained In the operation of the structure and hydraulic system shown in Figure 1, the first step is to clamp the sheet metal workpiece 1 in place, using only the proper amount of pressure on the clamping jaws I6, 11, and either individually or in multiple controlled by the hand wheels 66, it to provide the necessary full or partial clamping effects throughout the periphery of the workpiece. The proper amount of clamping effect depends on the shape of the article being formed from the stretched metal, and may be tentatively computed and later found by experiment to provide the necessary slippage at those places along the periphery of the workpiece as will prevent any wrinkling or fracture of the stretched metal.

Assuming that the jaw members 20 have been properly located by manipulating the hand wheels l4 and that fluid of predetermined pressure has been admitted at the right of the clamping pistons 26, the edge portions of the workpiece l are caused to be crimped by the movable jaws of the blocks ll coming into registry with the stationary mating blocks It. The push-button 55 is then depressed to cause pressure fluid to be admitted to the lower surface of the main ram 4, and the platen 5 is urged upwardly to a predetermined height to bend and to stretch the workpiece I into the desired shape, which is controllable not only by the shape of the upper surface of the platen 6 but also by the amount of slippage that is permitted at the various clamping cylinders. Obviously, any complex form of article may be produced by the upward movement of the ram 6 traveling, if necessary, at different speeds, and providing full clamping effects at one or more portions of the edges of the workpiece and only partial efiects or slippage at other portions of the workpiece. When the article has been formed the reversing valve 49 is operated to cause the platen S to be retracted and the relief valve 51. is opened to by-pass the clamping cylinders 26, which permits the formed article to be removed from the clamping jaws.

It has been pointed out, particularly in connection with Figure 2, that While the members I! may be operated entirely independently of one another and by their individual clamping cylinders, I prefer to connect the same together through a spring unit 3| (Figure 4). This unit tends to cause adjacent uneven clamping forces to be transmitted to the workpiece with smoothly graduated intensity.

Referring now to Figures 5 and 6, the ordinate of the graph represents pounds per square .inch exercised at the clamping cylinders, and the vertical dot-dash lines represent the center lines of the clamping cylinders. The light full line a represents the pressures exerted by the various clamping cylinders, the pressures of which are so regulated as to permit predetermined amounts of clamping or slippage effects. Whereas one group of cylinders may have applied thereto full fluid pressure, the next group of cylinders may have a lower pressure and the remaining group a still lower pressure. The pressure line a thus shows jogs b in its pressure line a where the pressure abruptly changes from one amount to another. This abruptness of change in pressure is bound to produce wrinkles and other inferior efiects in the stretched metal article. However, by the use of the flexible connecting piece 3| sudden change in pressure is graduated, as indicated by the full line a, and there is no sharp change in pressure between adjacent portions of the workpiece during the stretching and clamping operations.

Referring now more particularly to Figures '7 and 8, it will be apparent that by the use of the corrugated jaw surfaces the jaws will engage the metal firmly about the rounded teeth, and will be separated by exactly the thickness of the metal except at the biting portion [8, Where the metal will be slightly indented. No matter how extreme the clamping pressure, the extent of indentation will always remain the same on account of the separation of the jaws by the thickness of the metal around the contours of the rounded teeth. The action of these teeth is to put a broad crimp or corrugation in the margin of the workpiece and thus to resist the pull of the press ram, but such crimping action may not be sufficient to effect positive clamping except by the application of extreme clamping pressure. In addition. there is needed the bite of the jaws, which by exerting a pinching action at l8 over a localized area, requires only relatively low pressure positively to prevent even the slightest slippage of the metal. It will be noted that not only does the bite assist the rounded teeth in clamping the workpiece, but is in turn, prevented from tearing the metal under the tension of the pressing stroke by the resistance offered at the teeth.

When slippage is desired, the clamping pressure is partially released and the jaws will separate very slightly until the bite barely indents the surface of the metal. Clamping pressure has thus been removed, both from the area of the rounded teeth and the point of the bite. It might appear offhand that the pull of the ram 5 is now concentrated on the biting portion which alone is engaging the metal from both sides. However, even though the rounded teeth have separated slightly, they still exert a protective drag upon the metal and prevent tearing along the bite. This is true because the metal must still pass through the serpentine path provided by the teeth, stretching from one turn to the next on a tangent and thus ofiering considerable resistance to the slip of the metal. A very close analogy to the action here described is the lowering of a heavy object on the end of a rope by passing the rope one or more turns around a horizontal pole, and gripping it behind this pole. The object is lowered at a controllable rate, the rope is protected at the point of gripping, and very little gripping support is required because of the binding action of the rope around the pole. The jaws described hereinbefore and the method of operating the same provide, at the will of the operator, either absolute clamping of the metal or complete control of slippage of the metal, both of which functions are obtained without tearing or other damage to the metal.

In order further to improve the action of the metal stretching press, these jaws may be operated either singly or in groups by the system shown in Figure 2, thus to increase the adaptability of the improved press to the fabrication of a large variety of workpieces having all sorts of complex shapes. Any twisting or wrinkling forces which might be exerted at the junction between a clamping and a slipping jaw may be graduated by the use of the spring bars 3|, which cause adjacent uneven clamping forces to be transmitted to the workpiece with a smoothly graduated intensity.

It will be understood that I desire to comprehend within my invention such modifications as come within the scope of the claims and the invention.

Having thus fully described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In a clamping device for a work press adapted to operate on sheet material, a plurality of clamping cylinders for holding the workpiece, means including some of said clamping cylinders for exerting a full clamping effect, means including others of said clamping cylinders for exerting a slipping clamping eflect on the workpiece, and means including strips of flexible material extending between said clamping cylinders for transmitting the different clamping efi'ects to the.

workpiece with a smoothly graduated intensity.

2.. In a clamping device for a work press adapted to operate on sheet metal, a plurality of clamping cylinders for holding the workpiece, one of said clamping cylinders exerting a full clamping effect on the sheet metal, an adjacent clamping cylinder exerting a slipping pressure on said metal, and means including strips of flexible material extending between said clamping cylinders for graduating the transfer of pressure from a position on the sheet metal directly under the full clamping cylinder to a position on the metal directly under the slipping pressure cylinder.

3. In a clamping device for a work press adapted to operate on sheet metal, a plurality of clamping cylinders having rams for holding the workpiece, one of said clamping cylinders exerting a full clamping effect on the sheet metal, an adjacent clamping cylinder exerting a slipping pressure on said metal, and means for graduating the transfer of pressure from a. position on the sheet metal directly under the full clamping cylinder to a position on the metal directly under the slipping pressure cylinder, said means comprising a flexible strap secured directly to the rams of said adjacent clamping cylinders.

4. A device for gradating different pressures applied to a workpiece by a plurality of adjacently positioned pressure plungers, said device comprising a plurality of spring bars which extend over a distance corresponding to the distance covered by the plungers, said bars being interleaved with spacer plates of shorter length than the bars and secured together, and means for securing said plates to said plungers in order flexibly to join the plungers together through said spring bars.

5. A device for gradating difierent pressures applied to a workpiece by a pluralit of adjacently positioned pressure plungers, said device comprising a plurality of spring bars which extend over a distance corresponding to the distance covered by the plungers, said bars being interleaved with spacer plates of shorter length than the bars and secured together, and means for securing said plates to said plungers in order flexibly to join the plungers together through said spring bars, said spring bars having a U-shaped cross section and said plates having a width as snugly to be received by the side flange portions of said bars.

OTTO M. SUMMERS. 

